Replace AddOrUpdate with GetOrAdd (#915)

Author: Pliner

Source/EasyNetQ/Consumer/HandlerCollectionPerQueueFactory.cs

@@ -1,18 +1,15 @@
-
-using System.Collections.Concurrent;
+using System.Collections.Concurrent;
 using EasyNetQ.Topology;
 
 namespace EasyNetQ.Consumer
 {
     public class HandlerCollectionPerQueueFactory : IHandlerCollectionFactory
     {
-        readonly ConcurrentDictionary<string, IHandlerCollection> handlerCollections = new ConcurrentDictionary<string, IHandlerCollection>();
+        private readonly ConcurrentDictionary<string, IHandlerCollection> handlerCollections = new ConcurrentDictionary<string, IHandlerCollection>();
 
         public IHandlerCollection CreateHandlerCollection(IQueue queue)
         {
-            return handlerCollections.AddOrUpdate(queue.Name,
-                queueName => new HandlerCollection(),
-                (queueName, existingCollection) => existingCollection);
+            return handlerCollections.GetOrAdd(queue.Name, _ => new HandlerCollection());
         }
     }
-}
+}